docs: def accessible(self,fromstate, tostate):
          """Is state tonode directly accessible (in one step) from state fromnode? (i.e. is there an edge between the nodes). If so, return the probability, else zero"""
          if (not (fromstate in self.nodes)) or (not (tostate in self.nodes)) or not (fromstate in self.edges_out):
              return 0
          if tostate in self.edges_out[fromstate]:
              return self.edges_out[fromstate][tostate]
          else:
              return 0